How to convert uitextview content to a image in custom size?

I create a uitextview and I can add text and images to it with NSAttributedString and NSTextAttachment

  self.tvContent = [[UITextView alloc] initWithFrame:CGRectMake(0, 20, 
  self.view.frame.size.width, self.view.frame.size.height - 64)];
  self.tvContent.font = [UIFont fontWithName:@"Arial" size:16.0f];
  self.tvContent.backgroundColor = [UIColor whiteColor];
  self.tvContent.textContainerInset = UIEdgeInsetsMake(10, 10, 10, 10);
  self.tvContent.contentInset = UIEdgeInsetsMake(0, 0, 10, 0);
  self.tvContent.layoutManager.allowsNonContiguousLayout=NO;
  [self.view addSubview:self.tvContent];

And I convert its content to image with following code

  - (UIImage *)imageFromView:(UITextView *)view {
    CGRect tmpFrame = self.view.frame;
    CGRect aFrame = view.frame;
    aFrame.size.height  = [view sizeThatFits:[[UIScreen mainScreen] bounds].size].height;
    view.frame = aFrame;
    UIGraphicsBeginImageContext([view sizeThatFits:[[UIScreen mainScreen] bounds].size]);
    CGContextRef resizedContext = UIGraphicsGetCurrentContext();
    [view.layer renderInContext:resizedContext];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    view.frame = tmpFrame;
    return image;
 }

This image's max-width is 375(iPhone 6), if I wrote less content, such as 1234, the image's width is 40, so how can I convert the content to image with custom size, such as the width is 750, and the height is as long as the content height? Any ideas? many thanks.

The UITextView only renders within it's current frame, so you need to either resize the frame to the desired size before rendering, or create a temporary UITextView object to use for rendering purpose.

Here is how rendering with a temporary object can be done:

// Your call to get the image
    UIImage *image = [self imageFromTextView:self.tvContent width:750.0]; // Any width you want

// Your text view properties that also can be used for rendering purpose
- (void)setupTextViewProperties:(UITextView *)textView {
    textView.font = [UIFont fontWithName:@"Arial" size:16.0f];
    textView.backgroundColor = [UIColor whiteColor];
    textView.textContainerInset = UIEdgeInsetsMake(10, 10, 10, 10);
    textView.contentInset = UIEdgeInsetsMake(0, 0, 10, 0);
    textView.layoutManager.allowsNonContiguousLayout=NO;
}

Set up of the temporary UITextView object used for rendering.

- (UIImage *)imageFromTextView:(UITextView *)view width:(CGFloat)desiredWidth {
    // Create a new temporary UITextView to use for rendering
    CGRect frame = CGRectMake(0.0, 0.0, desiredWidth, 0.0);
    UITextView *temporaryTextView = [[UITextView alloc] initWithFrame:frame];

    // Setup the temporary text view
    // You can have different font, colors etc from the original view
    [self setupTextViewProperties:temporaryTextView];
    temporaryTextView.attributedText = view.attributedText;

    // Calculate the height needed for content with width
    CGSize size = CGSizeMake(desiredWidth, 0.0);
    size = [temporaryTextView sizeThatFits:size];

    // Resize the temporary view
    // Note that the calculated size can contain a width smaller that desired with
    temporaryTextView.frame = CGRectMake(0.0, 0.0, desiredWidth, size.height);

    // Get the image
    return [self imageFromView:temporaryTextView];
}

Helper that can render any view as an image.

- (UIImage *)imageFromView:(UIView *)view {
    CGSize size = view.bounds.size;
    UIGraphicsBeginImageContextWithOptions(size, YES, 0.0);
    CGContextRef context = UIGraphicsGetCurrentContext();
    [view.layer renderInContext:context];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return image;
}